The human heart has four major valves which moderate and direct blood flow in the cardiovascular system. These valves serve critical functions in assuring a unidirectional flow of an adequate blood supply through the cardiovascular system. The mitral valve and aortic valve control the flow of oxygen-rich blood from the lungs to the body. The mitral valve lies between the left atrium and left ventricle, while the aortic valve is situated between the left ventricle and the aorta. Together, the mitral and aortic valves ensure that oxygen-rich blood received from the lungs is ejected into systemic circulation. The tricuspid and pulmonary valves control the flow of oxygen-depleted blood from the body to the lungs. The tricuspid valve lies between the right atrium and right ventricle, while the pulmonary valve is situated between the right ventricle and the pulmonary artery. Together the tricuspid and pulmonary valves ensure unidirectional flow of oxygen-depleted blood received from the right atrium towards the lungs.
Heart valves are passive structures composed of leaflets that open and close in response to differential pressures on either side of the valve. The mitral valve acts as the inflow valve to the left side of the heart. Blood flows from the lungs, where it absorbs oxygen, and into the left atrium. When the mitral valve opens, blood flows from the left atrium to the left ventricle. The mitral valve then closes to prevent blood from leaking back into the lungs when the ventricle contracts to pump blood out to the body. Whereas the aortic, pulmonary, and tricuspid valves have three leaflets, the mitral valve has only two leaflets.
These heart valves may be rendered less effective by congenital, inflammatory, or infectious conditions, or disease, all of which may lead to dysfunction of the valves over time. Such degradation may result in serious cardiovascular compromise or even death. Because the left ventricle drives systemic circulation, it generates higher pressures than the right ventricle, and accordingly the aortic and mitral valves are more susceptible to dysfunction, such as stenosis or regurgitation. A stenotic mitral valve may impede blood flow into the heart, causing blood to back up and pressure to build in the lungs. Consequently, the presence of a stenotic valve may make it difficult for the heart to increase the amount of blood pumped during exercise, producing shortness of breath under physical activity. Regurgitation occurs when the mitral valve leaflets do not coapt correctly, thus causing blood to leak backwards into the left atrium and lungs each time the heart pumps. Improper coaptation of the mitral valve leaflets thus requires the heart to pump more blood with each contraction to eject the necessary amount of blood for systemic circulation; a process called volume overload. Although the heart may compensate for this overload for months to years, provided the progression of the leakage comes gradually, the heart will eventually begin to fail, producing shortness of breath and fatigue. Mitral valve dysfunction is rarely caused by congenital conditions, but is largely the result of degenerative disease due to advancing age, disease, or infection.
Previously known medical treatments to address diseased valves generally involve either repairing the diseased native valve or replacement of the native valve with a mechanical or biological valve prosthesis. All previously-known valve prostheses have some disadvantages, such as need for long-term maintenance with blood thinners, the risk of clot formation, limited durability, etc. Accordingly, valve repair, when possible, usually is preferable to valve replacement. However, most dysfunctional valves are too diseased to be repaired using previously know methods and apparatus. Accordingly, a need exists for a prosthesis capable of assisting heart valve function that enables treatment of a larger patient population, while reducing the need to fully supplant the native heart valve.
For many years, the standard treatment for such valve dysfunction called for surgical repair or replacement of the valve during open-heart surgery, a procedure conducted under general anesthesia. An incision is made through the patient's sternum (sternotomy), and the heart is accessed and stopped while blood flow is rerouted through a heart-lung bypass machine. When replacing the valve, the native valve is excised and replaced with either a mechanical or biological prosthesis. However, these surgeries are prone to many complications and long hospital stays for recuperation.
More recently, transvascular techniques have been developed for introducing and implanting a replacement valve, using a flexible catheter in a manner less invasive than open-heart surgery. In such techniques, a replacement valve is mounted in a crimped state at the end of a flexible catheter, and then advanced through the blood vessel of a patient until the prosthetic valve reaches the implantation site. The valve then is expanded to its functional size at the site of the defective native valve, usually by inflating a balloon within where the valve has been mounted. By expanding the prosthetic valve, the native valve leaflets are generally pushed aside and rendered ineffective. Examples of such devices and techniques, wherein the native valve is replaced in its entirety by a substitute tissue valve, are described, for example, in U.S. Pat. Nos. 6,582,462 and 6,168,614 to Andersen et al.
Mitral valve repair has become increasingly popular due to its high rates of success and the clinical improvements noted after repair. Several technologies have been developed to make mitral repair less invasive. These technologies range from iterations of the Alfieri stitch procedure; to coronary sinus-based modifications of mitral anatomy; to subvalvular placations or ventricular remodeling devices, which also may be employed to correct mitral valve regurgitation. Unfortunately, for a significant percentage of patients, mitral valve replacement is still necessary due to stenosis or anatomical limitations, and few less-invasive options are available for replacement procedures.
Prostheses have been produced and used for over forty years to treat cardiac disorders. They have been made from a variety of materials, both biological and artificial. Mechanical or artificial valves generally are made from non-biological materials, such as plastics or metals. Such materials, while durable, are prone to blood clotting and thrombus formation, which in turn increases the risk of embolization and stroke or ischemia. Anticoagulants may be taken to prevent blood clotting that may result in thromboembolic complications and catastrophic heart failure, however, such anti-clotting medication may complicate a patient's health due to the increased risk of hemorrhage.
In contrast, “bio-prosthetic” valves are constructed with leaflets made of natural tissue, such as bovine, equine or porcine pericardial tissue, which functions very similarly to the leaflets of the natural human heart valve by imitating the natural action of the heart valve leaflets, coapting between adjacent tissue junctions known as commissures. The main advantage of valves made from tissue is they are not as prone to blood clots and do not absolutely require lifelong systemic anticoagulation. A major disadvantage of tissue valves is they lack the long-term durability of mechanical valves. This is so because naturally occurring processes within the human body may stiffen or calcify the tissue leaflets over time, particularly at high-stress areas of the valve such as at the commissure junctions between tissue valve leaflets and at the peripheral leaflet attachment points, or “cusps,” at the outer edge of each leaflet. Furthermore, valves are subject to stresses from constant mechanical operation within the body. In particular, the leaflets are in tension when in a closed position and are in compression when in an open position. Such tension causes prosthetic valves to wear out over time, requiring replacement.
In recent years, bio-prosthetic valves have been constructed by integrating valve leaflets made from natural tissue into the stent-like supporting frame, which provides a dimensionally stable support structure for the valve leaflets. In more advanced prosthetic heart valve designs, besides providing dimensionally stable support structure for the valve leaflets, the stent-like supporting frame also imparts a certain degree of controlled flexibility, thereby reducing stress on the leaflet tissue during valve opening and closure and extending the lifetime of the leaflets. In most designs, the stent-like supporting frame is covered with a biocompatible cloth (usually a polyester material such as Dacron™ or polytetrafluoroethylene (PTFE)) that provides sewing attachment points for the leaflet commissures and leaflets themselves. Alternatively, a cloth-covered suture ring may be attached to the stent-like supporting frame, providing a site for sewing the valve structure in position within the patient's heart during a surgical valve replacement procedure.
While iterative improvements have been made on surgical bioprosthetic valves over the last several decades, existing bioprosthetic valves still have drawbacks. One drawback is the mismatch in size and mass between opposing surfaces of the stent-like supporting frame. The mismatch is often due to the variability in the shapes and mechanical characteristics of the stent-like supporting frame. For prosthetic valves with balloon-expandable stent-like supporting frames, the recoil of the supporting frames post-balloon-inflation may lead to perivalvular leaks around the circumference of the prosthetic valve and potential slippage and migration of the valve post-implantation. Another risk associated with prosthetic valves having balloon-expandable supporting frames is potential damage to the leaflets of the prosthesis during implantation, when the leaflets may be compressed between the balloon and the supporting frame. For prosthetic valves with self-expanding stent-like supporting frames, mismatch may arise due to the deformation/movement of the supporting frame, e.g., slight deformation of the frame into a less than circular shape during normal cardiac movement. Such mismatch may lead to instability among components of a prosthetic valve, resulting in perivalvular leaks and uneven stress distribution in the valve leaflets, resulting in accelerated wear of the valve.
Another drawback in the construction of existing bio-prosthetic valves with self-expanding supporting frames is the potential for damage to the leaflet tissue arising from the spacing between the struts of the frame. For example, when the stent-like supporting frame is deployed, the distance between struts during expansion of the frame may stretch both the surrounding tissue and the leaflet tissue further apart than designed, potentially resulting in damage to surrounding tissue and leaflet tissue. With use of an oblong or circular radially self-expanding frame applied on the majority of the mitral valve, there is risk of left-ventricular outflow tract (LVOT) obstruction.
A mitral valve regurgitation often arises due to mitral annulus dilatation, which may be treated using a surgical technique to narrow and restore the natural shape the annulus. Usually the mitral valve and annulus are shaped like a “D”, but when dilated the shape becomes more like an “O”. Prosthetic annuloplasty rings are therefore an important additional component in some mitral valve repair techniques. A primary role of the annuloplasty ring is to reduce the size of the annulus and decrease the tension on the sutures while providing flexibility and mobility at the same time. An annuloplasty ring thus is omitted during mitral valve repair only exception in cases of infective endocarditis, in order to avoid excess foreign material. When an annuloplasty ring is used, three months of anticoagulation is often prescribed.
One recent technique for correcting mitral valve leakage, as described for example in U.S. Pat. No. 6,269,819 to Oz et al., employs a percutaneously placed catheter to introduce a clipping apparatus into a leaking mitral valve. Once positioned, the clip arms are unfolded and advanced into the left ventricle below the valve leaflets, after which it is retracted and closed over the leaflets, holding them together to reduce mitral regurgitation. If further improvements to regurgitation are to be made, the clip is released and further advanced for repositioning. Once decrease of leakage has been assessed, the clip is deployed to entrap together the free edges of the mitral leaflets, and the catheter withdrawn. The clip may be made of metal with a polyester fabric covering to promote healing. Because the clip transforms the mitral orifice into two orifices, the clip may significantly obstruct the flow of blood through the valve.
In view of the above-noted drawbacks of previously-known systems, it would be desirable to provide a device, and methods of using the same, that assists the functioning of the native cardiac valve, rather than removing or entirely supplanting the native valve. The native structures (mitral leaflets, chordae, papillary muscles, etc.) play an important role in left-ventricular function and therefore any valve replacement system that does not respect these elements may adversely impact the left-ventricular function.
It would also be desirable to provide a device having prosthetic leaflets, and methods of using the same that reduces tension on the prosthetic leaflets, thereby increasing the life of the prosthesis.
It further would be desirable to provide a device having a support frame, and methods of using the same, wherein the prosthesis is configured to firmly anchor to the native valve when deployed, without deformation or movement of the supporting frame at the annulus, thus reducing the risk of perivalvular leakage.
It still further would be desirable to provide a device, and methods of using the same, that may be deployed with reduced risk of obstructing blood flow relative to previously known mitral valve repair techniques.